There are presently many different types of bone plate systems for securing bones so that the secured bones may fuse or heal. As used herein, the term bone may refer to a bone, a bone fragment, or a portion of a bone. Bone plate systems often utilize a bone plate having throughbores and bone screw assemblies that are driven through the throughbores and into the underlying bone for securing the bone plate to one or more bones. In some applications, the bone screw assemblies include a bone screw that is driven into bone and a device associated with the bone screw for restricting back-out of the bone screw once the bone anchor assembly has been seated in the throughbore of the bone plate. The device may include a resilient head of the bone screw for being seated within one of the throughbores and a set screw that is threaded into the resilient head to expand the head against walls of the throughbore and restrict back-out of the bone screw. However, the resilient head is weaker than the remainder of the bone screw in order to permit expansion of the head as the set screw is threaded into the head. Further, the set screw is relatively small and may be difficult to thread into the bone screw head during surgery.
Another type of bone plate system utilizes bone screws having heads with resilient c-rings carried thereon and locking members disposed within the bone screws for restricting back-out of the bone screws from throughbores of the bone plate. Once the heads of the bone screws have been seated within the throughbores, the locking members are longitudinally shifted within the bone screw to radially expand the c-ring into engagement with walls of the throughbore. The c-ring has an outer annular portion for engaging the throughbore walls and inner, radially extending portions configured to contact the locking member and shift the outer annular portion radially outward with longitudinal shifting of the locking member. However, the radially extending portions are thin and may deflect when loads are applied to bone screw, such as post-operation movement of the patient. Deflection of the radially extending portions may, over time, reduce the strength of radially extending portions and the overall stability of the bone plate system.
In some instances, a predetermined amount of pivoting between a bone screw and a bone plate of a bone plate system is desired to accommodate settling of the bones. The pivoting is preferably controlled movement, rather than free movement between the bone screw and bone plate which may interfere with fusion of the bones. Although bone screws having c-ring back-out prevention devices may be used in these applications, the c-rings themselves have an outer surface for engaging the bone plate throughbore walls that is relatively thin compared to the bone screw head, e.g., less than a quarter of the height of the bone screw head. The short vertical extent of the c-ring outer surface along the throughbore wall limits the contact area and frictional engagement between the c-ring outer surface and the throughbore wall. This is undesirable in some instances because the limited frictional engagement provided by the c-ring limits the ability of the expanded c-ring to control pivoting of the bone screw relative to the bone plate.
Another shortcoming of prior bone plate systems is the ability to use a single bone plate system for a variety of patient anatomies. For example, to stabilize a pair of vertebrae, an intervertebral implant is inserted between the vertebrae and a bone plate system is connected to the vertebrae to secure the vertebrae and the intervertebral implant together. The intervertebral implant may be selected from a number of different shapes and sizes to conform to the patient's anatomy. Due to the possible variation in the intervertebral implant selected for a particular patient, the bone plate system subsequently used to secure the bones should accommodate the range of shapes and sizes of the intervertebral implant that may be used. One prior approach to providing such a bone plate system utilized a bone plate having elongated throughbores. The bone plate is first positioned on a pair of vertebrae stabilized by an intervertebral implant, and then bone screws are driven into the elongated throughbores at locations along the throughbores that permit the bone screws to engage the underlying vertebrae. Although the elongated throughbores provide flexibility in installation, the bone screws can slide along the elongated throughbore as the vertebrae settle. In some instances, this post-operative sliding of the bone screws is undesirable due to the corresponding changes in position of the vertebrae.